Among optical systems using existing optical devices (such as convex lenses) in order to form the image of an object and then to transfer the image to a separate location and form there another image, for instance the relay lens system might be mentioned as a representative example (see for example Non-Patent Reference No. 1). In case of the relay lens system, it is customary to choose a configuration where in addition to the objective lens facing the object to be projected, several other lenses are also included and arranged to form further images of the object to be projected, so that the image might be observed in a location separated from the object to be projected and from the objective lens, and this principle is used for instance as rigid scopes for endoscopes (see for instance Non-Patent Reference No. 2).
Furthermore, imaging optical devices have been proposed comprising of a symmetry surface forming a plane (optical device plane) such that light paths of light passing through the plane are bent, with the property that the formed image will be reversed in the depth direction with respect to the plane. It is known that by the proper arrangement such imaging optical systems are capable of forming real mirror images, in which case they are capable of forming not only two-dimensional images but also three-dimensional images without distortion. One example of the above-mentioned imaging optical systems is the afocal lens array (see Patent Reference No. 2) making use of the refraction of light, by using optical devices with infinite focal distance, that is afocal devices, as unit optical elements.
Another imaging optical system as proposed by the present inventor makes use of the effect of reflection of light by specular surfaces, by arranging one or more specular surfaces perpendicularly or nearly perpendicularly to the optical device plane to serve as unit optical elements reflecting light (dihedral corner reflectors) thus forming a dihedral corner reflector array (see Patent Reference No. 1). The dihedral corner reflector array has the property that light emitted from an object to be projected (including objects with physical reality and displayed images) that is placed to one side of the optical device plane, will be transmitted through the dihedral corner reflector array by being reflected by each specular surface of the unit optical elements, thus forming a real image of the object to be projected in the space on the other side of the optical device plane.
Still another imaging optical device proposed by the present inventor comprises of a retroreflector with recursive reflecting capability, and a half-mirror (see Patent Reference No. 3). The above-mentioned imaging optical device operates by recursively reflecting light emitted by the object to be projected and reflected by the half-mirror, then passing the light through the half-mirror; or alternately, by recursively reflecting light emitted by the object to be projected and passed through the half-mirror, then reflecting the light by the half-mirror; in either case, the half-mirror surface functioning as the plane of symmetry, whereby a mirror image of the object to be projected is formed as a real image at a planar symmetric position without distortion; therefore similarly to a dihedral corner reflector array making it possible to observe two-dimensional real images in case the object to be projected is two-dimensional, or three-dimensional real images in case the object to be projected is three-dimensional, from an oblique direction.
Patent Reference No. 1: WO2007/116639
Patent Reference No. 2: JP 2005-10755
Patent Reference No. 3: JP 2009-025776
Non-Patent Reference No. 1: “Colum-5 relay lens” (“Colum-5 rire-renzu”), by Shin-Ichi Nagata, in: “Understand Lenses Illustrated” (“Zukan renzu ga wakaru hon”), Nippon Jitsugyo Shuppansha Co., Ltd., Nov. 20, 2002, p. 118 (in Japanese)
Non-Patent Reference No. 2: “IV. Optical Devices” (“IV. Kougaku kiki”), by Shin-Ichi Nagata, in: “Latest Optical Technology Handbook” (“Saishin kougaku gijutsu handobukku”), Asakura Shoten Co., Ltd., Sep. 29, 2002, pp. 830-831 (in Japanese)